1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image that has been formed on a latent image carrier, a process cartridge that integrally comprises this developing device, and an image forming apparatus, such as a multifunctional machine, which comprises at least one of a copier, printer, facsimile machine and plotter that comprises either this developing device or process cartridge.
2. Description of the Related Art
The developing device used in a copier, printer, facsimile machine and other such image forming apparatuses to date has been either a two-component development system or a one-component development system. The two-component development system is extremely well suited to high-speed developing, and is the mainstream system for present-day medium-speed and high-speed image forming apparatuses.
In the two-component development system, the developer on the contact part of the electrostatic latent image on the latent image carrier must be in an extremely dense state in order to strive for high quality. For this reason, efforts to make carrier particles smaller are currently being pushed forward, and carriers of around 30 μm are coming into use at the commercial level.
The one-component development system is currently the mainstream system for low-speed image forming apparatuses due to the fact that the mechanism is compact and lightweight. In the one-component development system, a blade, roller and other such toner regulating members are allowed to make contact with the toner on the development roller to form a thin layer of toner on the development roller, and the toner is electrostatically charged at this time by the friction between the development roller, toner regulating members and the toner. The charged toner layer, which is thinly formed on the development roller, is transported to the development area, and develops a charged latent image on the latent image carrier. The development mode here is broadly divided into a contact type and a non-contact type, the former being a mode in which the development roller and latent image carrier make contact with one another, and the latter being a mode in which the development roller and latent image carrier do not make contact.
To make up for the deficiencies of the above-mentioned two-component development system and one-component development system, a number of hybridized systems that combine a two-component development system and a one-component development system have been proposed, as disclosed, for example, in Japanese Patent Application Laid-open No. H3-100575 (Prior Art 1).
As a method for developing tiny, uniform, high-resolution dots, for example, there is the system disclosed in Japanese Patent Application Laid-open No. H3-113474 (Prior Art 2). In contrast to the above-mentioned hybridized system, this system creates a toner cloud in the development area and realizes the developability of high-resolution dots by installing a wire that applies a high-frequency bias to the development area.
Further, Japanese Patent Application Laid-open No. H3-21967 (Prior Art 3) proposes a method for forming an electric field curtain on a rotating roller to form the most efficient and stable toner cloud.
Further, Japanese Patent Application Laid-open No. 2003-15419 (Prior Art 4) discloses a developing device that transports the developer via an electric field curtain in accordance with a traveling wave field.
Further, Japanese Patent Application Laid-open No. H9-269661 (Prior Art 5) discloses a developing device having a plurality of magnetic poles, which nearly uniformly clamps nearly one layer of carrier to the circumferential surface of the development roller.
Further, Japanese Patent Application Laid-open No. 2003-84560 (Prior Art 6) discloses a developing device that disposes via an insulating part a periodic conductive electrode pattern on the surface of the developer carrier, which carries a non-magnetic toner, generates an electric field gradient in the vicinity of the surface of the developer carrier by applying a prescribed bias potential to these electrodes, thereby adhering and transporting the above-mentioned non-magnetic toner on the above-mentioned developer carrier.
The demand for high image quality is becoming increasingly higher for the two-component development system, and the required pixel dot size itself must be either the same or smaller than the diameter of the current carrier particles. Therefore, from the standpoint of discrete dot reproducibility, carrier particles must be made even smaller.
However, as the size of the carrier is made smaller, the magnetic permeability of the carrier particles declines, increasing the likelihood that the carrier will separate from the development roller. When the separated carrier particles adhere to the latent image carrier, not only does the adherence of the carrier itself give rise to image defects, but various other side effects also occur as a result of this, such as damage to the latent image carrier.
To prevent carrier separation, attempts are being pushed forward on the material side to raise the magnetic permeability of the carrier particles, and efforts are also being made to strengthen the magnetic force of the magnet embedded inside the development roller, but the need to reduce costs while raising image quality is making development extremely difficult.
Further, as the diameter of the development roller becomes increasingly smaller in response to the trend toward miniaturization, it is becoming difficult to design a development roller that has a magnetic field configuration powerful enough to completely suppress carrier separation.
To begin with, since the two-component development system is a process that forms a toner image by rubbing the rests of the two-component developer, called the magnetic brush, against the electrostatic latent image, the unevenness of the crests inevitably gives rise to irregularities in the developability of discrete dots.
It is possible to enhance image quality by forming alternating electric fields between the development roller and the latent image carrier, but it is difficult to completely do away with basic image irregularities, such as the irregularities of the crests of the developer.
Further, in order to enhance transfer efficiency and cleaning efficiency in the step for transferring a toner image that has been developed on the latent image carrier, and the step for cleaning the residual toner left on the latent image carrier subsequent to transfer, the non-electrostatic adhesion between the latent image carrier and the toner must be reduced as much as possible. As a method for lowering the non-electrostatic adhesion between the latent image carrier and the toner, reducing the friction coefficient of the surface of the latent image carrier is known to be effective, but, since the crests of the two-component developer slip smoothly through the development area in this case, development efficiency and dot reproducibility become extremely poor.
In the one-component development system, a layer of toner on the development roller that has been thinned by the toner regulating members makes full press-contact with the development roller, thereby causing the toner responsiveness to the electric field of the development area to become extremely poor. Accordingly, in order to normally achieve high image quality, the mainstream approach is to form a powerful alternating electric field between the development roller and the latent image carrier, but even with the formation of this alternating electric field, it is difficult to stably develop a fixed amount of toner for an electrostatic latent image, and it is difficult to uniformly develop a tiny, high-resolution dot.
Further, since the one-component development system applies an extremely high stress to the toner when forming the thin layer of toner on the development roller, the toner circulating inside the developing device deteriorates extremely rapidly. In line with the deterioration of the toner, irregularities and the like become more likely even in the process for forming the thin layer of toner on the development roller, making the one-component development system unsuitable for high-speed or high-durability image forming apparatuses.
A hybridized system overcomes a number of problems even though the size and number of parts of the developing device itself increase. However, in the end, the development area is still faced with the same problem as that of the one-component development system, that is, developing a tiny, uniform, high-resolution dot is still difficult.
The system disclosed in Prior Art 2 is able to realize highly stable, high image quality development, but the complexity of the developing device configuration cannot be avoided.
The system disclosed in Prior Art 3 can be said to be extremely good at achieving compact size and high image quality development, but as a result of the diligent research of the inventors, it was discovered that the conditions for development and for the electric field curtain that is formed must be strictly limited in order to achieve ideal high image quality. That is, if image creation is carried out using a condition that strays from the appropriate condition, the effectiveness of this system is completely lost, resulting in inferior image quality instead.
Now then, in an image creation process such that a first toner image is formed on the latent image carrier, and a second toner image and third toner image are formed in order thereon, the development system must be one that does not disturb the toner image first formed on the latent image carrier.
It is possible to sequentially form toners of respective colors on the latent image carrier by using a non-contact one-component development system or the toner cloud development system disclosed in Prior Art 2, but since an alternating electric field is formed between the latent image carrier and the development roller in both systems, a portion of the toner is pulled away from the toner image first formed on the latent image carrier, and enters the developing device. Consequently, not only is the image on the latent image carrier disturbed, but there also arises the problem of different colored toners being mixed together inside the developing device. It is crucial that these systems achieve high quality images, and to solve for this problem will require a method that realizes toner cloud development without forming an alternating electric field between the latent image carrier and the development roller.
As a method that is capable of realizing toner cloud development like this, the system disclosed in Prior Art 3 cited above is conceivably effective, but as mentioned above, this system is completely ineffective unless used under the appropriate conditions.
Further, a system such as that disclosed in Japanese Patent Application Laid-open No. 2002-341656 (Prior Art 7) is also a conceivably effective method for electrostatically transporting and developing the toner using an alternating electric field of three or more phases without driving the toner carrier mechanically.
However, the problem posed by this method is that, if for one reason or another, the toner can no longer be transported electrostatically, this toner accumulates on top of the transport substrate, resulting in a loss of functionality.
To solve for this problem, for example, a structure that combines a fixed transport substrate with a toner carrier that moves along the surface thereof has also been proposed, as in the system disclosed in Japanese Patent Application Laid-open No. 2004-286837 (Prior Art 8), but the mechanism becomes extremely complex.